In the field of lighting, there are a number of applications that call for lighting systems that illuminate large surface areas. In many of these lighting systems, it is desirable to have the distance between the light source and the illuminated object as small as possible. It is therefore desirable to provide a uniform illumination system that is thin and that can be made at an acceptable cost.
One such lighting system, commonly referred to as an edge lit system, uses a light guide to distribute light, introduced into the light guide at an edge of the light guide, over a relatively large surface area. Such systems are used commonly as backlights for display devices such as liquid crystal display (LCD) panels. Such display devices are commonly used today for a large variety of applications ranging from televisions, computer displays and handheld devices. One of the principal reasons for the popularity of LCD display panels is their small size and thickness and relatively low power consumption.
Conventional edge lit light sources typically use a tubular lamp such as a cold-cathode fluorescent lamp (CCFL.). Light emitted by a CCFL is typically coupled into a solid light guide, which forms a light-emitting panel. The surface area of the light-emitting panel usually corresponds to the area of a display panel to be illuminated. The light-emitting panel typically has one surface (an illumination surface) through which light is extracted from the light guide.
In recent years, alternative light sources have been developed and explored in an effort to provide brighter more efficient lighting. One example of such alternative light sources are point light sources, such as light-emitting diodes (LEDs). There is increased effort to use such light sources in place of the CCFL light sources. While a number of advantages are associated with LED light sources, they present new challenges for distributing the light uniformly over a surface area. Additionally, using currently available LEDs, it is often desirable to use a combination of LEDs of different colors to provide white light. In such a system, the different colors of light must be efficiently mixed to provide uniform, white light.
One example of an LED-based light source is illustrated in FIG. 1. A light-emitting panel 1 has a front wall 2 and an opposed rear wall 3. The panel 1 is bound by end surfaces 4, 5. At least one of the end surfaces 4 is light transmitting. The backlight system has a light source 6 such as multiple LEDs. Light from the light source 6 is coupled into the light-emitting panel 1 through the edge surface 4. The panel 1 is wedge shaped such that the light-emitting panel increases in thickness in a direction away from the light source 6. This light source is more fully described in published Patent WO 03\048635. Such a system relies on frustrated total internal reflection (TIR) to extract light from the light guide. As will be appreciated from the description of the present invention, it is difficult to extract light out of such a system in a spatially uniform manner. There remains a continuing need to provide improved surface area light sources that efficiently distribute light and are small in size and low cost.